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Pettenò: Debunking x32 mythsPettenò: Debunking x32 mythsPosted Jun 25, 2012 21:45 UTC (Mon) by mansr (guest, #85328)In reply to: Pettenò: Debunking x32 myths by Flameeyes Parent article: Pettenò: Debunking x32 myths
>> This makes a big difference on some applications, on the order of 40% on both Atom and Core i7, as demonstrated by non-trivial benchmarks run by the x32 ABI developers.
> > Where are you reading that 40%?
There is a brief mention on https://sites.google.com/site/x32abi/ of a 40% improvement in 181.mcf from spec2k.
This is a pointer chasing benchmark where the entire data set consists of pointers, or hardly a realistic test case. It is furthermore an amazingly poorly written piece of software. In many places it needlessly uses global variables in inner loops, which can thwart compiler optimisations (particularly aliasing-based ones). It also uses 'long' exclusively where any sane programmer would use 'int', possibly leading to more expensive 64-bit operations being used where there really is no need.
Moreover, the website does not mention which compiler, let alone which compiler flags, were used, nor does it provide any raw numbers from the benchmark run. A lone percentage figure as presented there means absolutely nothing whatsoever.
Finally, out of all the spec2k modules, they chose to showcase only two, presumably because those two showed the most favourable results. The second one is 186.crafty, showing a meagre 3% improvement (4% on Atom). This leaves one wondering what the results of the remainder looked like. Something tells me they showed improvements of less than 3%, if any at all.
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Pettenò: Debunking x32 myths Posted Jun 25, 2012 22:05 UTC (Mon) by butlerm (subscriber, #13312) [Link]
181.mcf doesn't sound like a toy benchmark to me:
"For the considered single-depot case, the problem can be formulated as a large-scale minimum-cost flow problem that we solve with a network simplex algorithm accelerated with a column generation. The core of the benchmark 181.mcf is the network simplex code 'MCF Version 1.2 -- A network simplex implementation', For this benchmark, MCF is embedded in the column generation process."
In any case, if 181.mcf is so poorly written, perhaps someone could do us the favor of benchmarking other pointer intensive code instead of dismissing x32 without bothering to conduct a single relevant benchmark.
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